State of the art methods for depositing a solder material on a substrate in the form of a predetermined pattern are for instance (electrical) plating, stencil printing or thermal evaporation.
Problems arise if a large variety of geometrical shapes and areas on the substrate require deposition of the solder material. This is typically the case in (hermetic) sealing-ring packages (see FIG. 1) that require a wide rim of solder material for mechanical stability and a small nozzle at one point of the rim to allow degassing or evacuation of the cavity. The nozzle can be realized by a small indent of the rim that is closed by a subsequent reflow of the solder (so-called indent re-flow sealing, see for instance in U.S. Pat. No. 6,297,072, entitled METHOD OF FABRICATION OF A MICROSTRUCTURE HAVING AN INTERNAL CAVITY, issued on Oct. 2, 2001, which is incorporated by reference in its entirety).
The problems stem from the non-uniform deposition of the solder material. Such non-uniformities introduce deviations from parallelism and impede the soldering process. The cause of this non-uniformity is two-fold:    1. Deposition by electro-plating (Sn/Pb-alloys and In) suffers from electric field-line crowding (i.e. current crowding) at edges. The deposition rates are therefore different for different geometries. If different shapes or sizes of the deposition areas are present on the same substrate there will be a local variation of the amount of deposited material.    2. As for evaporated material, small structures are difficult to define with the thick resists that are required for the lift-off of the thick soldering metal. In particular small structures of different sizes on the same substrate are difficult to define. The extreme case can be that small structures are not fully developed (they remain open).
Different geometries and sizes of solder areas are ubiquitous in wafer level packaging because various kinds of devices may be manufactured on a single wafer. Non-uniformities of the solder deposition impede further miniaturization of this packaging technique.
It is therefore desirable to find a method, which permits an application of the solder so as to adapt for the here mentioned non-uniformities. It would be even more preferable to have a method that can be used to provide the solder on locations on the substrate that are out of reach of the present solder deposition-techniques (electro-plating and thermal evaporation), i.e. into pitches and via-holes.
Typical solder materials today are for instance Sn/Pb. The use of Pb will be banned by law, so new alternatives are sought; these alternatives should preferably have good properties and/or should allow methods such that hermetic sealing is possible and a stable bond is formed (e.g. high shear force).
Embodiments of the present invention aim to provide a method, which permits a uniform distribution of the solder. It is a further aim of certain embodiments to provide a method which allows distribution of the solder to locations on the substrate/device that are out of reach of the present solder deposition-techniques (electroplating and thermal evaporation), e.g. into pitches and via-holes, or under bridge-like structures.